As the ability to reduce the physical size of semi-conductor circuits has increased, there exists a corresponding need to dissipate the power generated from these circuits. Effective cooling is required in situations involving high circuit density in order to maintain chip operating temperatures at acceptable levels. Therefore, there has been an ever increasing need to improve the heat transfer characteristics of semi-conductor packaging material.
Various techniques are available for measuring the thermal-characteristics of the packaging material used to package semi-conductor chips. One common technique is the use of a thermal test die as disclosed in Manual for Using Delco Electronics Thermally Sensitive Die, by D. L. Rodkey, Jan. 30, 1987. The apparatus disclosed within this manual relates to measuring the thermal resistance of semiconductor packaging material. In that apparatus, a test die consisting of an isolated diode, conductive traces, and heating elements are placed within semi conductor packaging material having the size and shape of a semi conductor chip. A fixed current is forced through the isolated diode, causing a fixed forward voltage drop across that diode. The heating element is then activated, causing the test die and the diode to heat up. As known to one skilled in the relevant art, the forward voltage drop of a diode changes in response to the temperature of that diode. For the apparatus disclosed in the Delco manual, the heat generated in the test die will cause a decrease in the forward voltage drop of the diode by 2 mV for each degree Celsius rise in temperature. The calculations from which this equation is derived are located in the Delco manual, page 3. Sensing pads are attached in parallel across the diode to measure the decrease in the forward voltage drop of the diode as due to the heat generated by the heating element. The better the thermal conductive properties of the semiconductor packaging material, the greater the power dissipation of that material, resulting in a lower decrease in the forward voltage drop of the diode.
A common problem which plagues this type of test die is its inaccuracy resulting from voltage drops caused by resistance in the trace material. In theory, the decrease in the forward voltage drop of the diode should be due purely to the rise in temperature of the diode. However, in reality, the voltage drop across the diode as measured at the sensing pads necessarily includes an error attributed to the resistance in the conductive trace. As commonly known to one skilled in the art, current passing through a resistive element will experience a voltage drop. Since a constant current is forced through the first set of conductive traces connecting the forcing pads to the diode, the resistance contained in the first set of traces results in a voltage drop along those traces. Thus, the sensing pads which are connected via a second set of conductive traces to the first set of conductive traces measure not only the forward voltage drop across the diode, but also the voltage drop due to the resistance in the first set of conductive traces. Depending upon the size of the die and the thickness of the conductive trace used, errors in excess of 44 percent have been calculated when measuring the forward voltage drop of the diode. This error ratio, known as thermal resistance measurement error, is attributable to the resistance in the first set of conductive traces which connect the forcing pads to the diode. The resistance contained in this first set of traces becomes significant because of the amount of current which is being forced through those traces.
Thus the thermal test die apparatus which is available for measuring the thermal characteristics of various semiconductor packaging materials has not proved to be as accurate as is desired.